ath9k: make relay callbacks const

Now that relay_open() accepts const callbacks, make relay callbacks
const.

Link: https://lkml.kernel.org/r/7db0286c428f3a478dd7544afef04a3b131f1aa0.1606153547.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Kalle Valo <kvalo@codeaurora.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/net/wireless/ath/ath9k/common-spectral.c

@@ -1053,7 +1053,7 @@ static int remove_buf_file_handler(struct dentry *dentry)
 	return 0;
 }
 
-static struct rchan_callbacks rfs_spec_scan_cb = {
+static const struct rchan_callbacks rfs_spec_scan_cb = {
 	.create_buf_file = create_buf_file_handler,
 	.remove_buf_file = remove_buf_file_handler,
 };